1. Field of the Invention
This invention relates to a frequency multiplier constructed to input a microwave or millimeter wave frequency band signal and output a multiplied output signal whose frequency is a multiple of that of the input signal, and to a voltage controlled oscillator necessary for the utilization of waves of these frequency bands.
2. Prior Arts
A frequency multiplier is constructed to output a signal (namely, a multiplied output signal) whose frequency is a multiple of that of an input signal by utilizing the nonlinearity of the input-output characteristic of a transistor or the like. In this construction, it is required that the power of the multiplied output signal be made large and that the power of unwanted output signals be made small. Here, unwanted output signals are low-order or high-order frequency signals that are signals other than the multiplied output signal in the output signal outputted from the transistor or other device. Among these unwanted output signals, the output signal having the same frequency as the frequency of the input signal is called the fundamental wave signal. The output power of this fundamental wave signal is often greater than the output power of the multiplied output signal, and for this reason a circuit is provided in the frequency multiplier for suppressing this fundamental wave signal so that it is not outputted.
This kind of frequency multiplier is basically made up of an input matching circuit, a transistor, a fundamental wave signal band suppressing circuit, and an output matching circuit. The input matching circuit is a circuit for providing matching with respect to the frequency of the input signal, and the output matching circuit is a circuit for providing matching with respect to the frequency of the multiplied output signal. For the fundamental wave signal band suppressing circuit, a reflecting type circuit is generally used which reflects the fundamental wave signal (unwanted signal).
There have been frequency multipliers of the 20 construction described above consisting of monolithic microwave integrated circuits (MMIC). An example of such a construction is the frequency multiplier shown in `A 60 GHz MMIC STABILIZED FREQUENCY SOURCE COMPOSED OF A 30 GHz DRO AND A DOUBLER`, 1995 IEEE Microwave Symp. Digest pp. 71-74. The construction of this frequency multiplier is shown in FIG. 7.
As shown in FIG. 7, a frequency multiplier 1 has an input matching circuit 2, a transistor 3, a reflecting type fundamental wave signal band suppressing circuit 4 and an output matching circuit 5. The input matching circuit 2 consists of a transmission line 6 and a stub 7. One end of a capacitor 8 is connected to a point of connection between the transmission line 6 and the stub 7, and the other end of this capacitor 8 is made an input terminal 9. The transistor 3 consists of, for example, an FET; its gate is connected to an end of the transmission line 6 of the input matching circuit 2 and is also grounded by way of a transmission line 10 and a capacitor 11.
The reflecting type fundamental wave signal band suppressing circuit 4 consists of an open stub 12. The output matching circuit 5 is made up of a transmission line 13 and a stub 14. The drain of the transistor 3 is connected to one end of the open stub 12 and to one end of the transmission line 13 and also is grounded by way of a transmission line 15 and a capacitor 16. A point of connection between the transmission line 13 and the stub 14 is connected to one end of a capacitor 17, and the other end of this capacitor 17 is made an output terminal 18. A point of connection between the transmission line 10 and the capacitor 11 is made a voltage terminal 19 through which a gate bias is supplied. A point of connection between the transmission line 15 and the capacitor 16 is made a voltage terminal 20 through which a drain bias is supplied.
In a frequency multiplier of the construction described above, because the multiplied output signal is generated utilizing the nonlinearity of the input-output characteristic of the transistor 3, the conversion gain of the frequency multiplier is generally very low; in other words, there is the characteristic that the output power of the multiplied output signal is low. Because of this, there has been a strong need for the conversion gain of frequency multipliers to be increased.
In this connection, the present inventors tried various experiments with the aim of increasing the conversion gain of a frequency multiplier. First, the inventors focused on the known characteristic that when the transistor is operated at a point at which the nonlinearity of the input-output characteristic of the transistor is greater the output power of the multiplied output signal increases. At that time, the inventors noticed that the transistor can be made to operate at a point at which the nonlinearity of the input-output characteristic of the transistor is greater by raising the high-frequency voltage acting on the output terminal of the transistor.
Here, as a method of raising the high-frequency voltage acting on the output terminal of the transistor, the method of connecting a voltage terminal for impressing a bias voltage from outside to the output terminal of the transistor is readily conceivable. However, when a voltage is impressed in this way, all that happens is that the direct current voltage level rises, and the effect of the high-frequency voltage rising intended by the inventors cannot be expected.
In view of this, the inventors looked for a method of raising the high-frequency voltage acting on the output terminal of the transistor without impressing a voltage from outside. The inventors focused on the fact that the fundamental wave signal outputted through the output terminal of the transistor is reflected by the reflecting type fundamental wave signal band suppressing circuit and then this reflected signal is reflected at the output terminal of the transistor, by such reflections of the signal being repeated, the (reflection) decay of the signal occurs in the part (the transmission line) connecting the output terminal of the transistor to the input terminal of the reflecting type fundamental wave signal band suppressing circuit. The inventors had the idea that it might be possible to raise the voltage acting on the output terminal of the transistor by utilizing this reflection, and as a result of pursuing this idea invented a construction wherein a transmission line having the function of producing a standing wave on the basis of the above-mentioned fundamental wave signal and its reflected signal is provided between the output terminal of the transistor and the input terminal of the reflecting type fundamental wave signal band suppressing circuit.
To confirm the operation of this construction, the inventors carried out the experiment of making an MMIC type of frequency multiplier wherein a transmission line having the function of producing a standing wave is provided between the output terminal of a transistor and the input terminal of a reflecting type fundamental wave signal band suppressing circuit. When the output power of the multiplied output signal outputted from this frequency multiplier and its conversion gain were measured, it was confirmed empirically that the output power and conversion gain had increased considerably. The specific construction and measurement results of this MMIC of the frequency multiplier are discussed in detail below in the section on preferred embodiments of the invention.
Accordingly, a first object of the present invention is to provide a frequency multiplier with which it is possible to increase the conversion gain and increase the output power of the multiplied output signal by adopting a construction such that the transistor operates at a point at which the nonlinearity of the input-output characteristic of the transistor is greater.
Now, when utilizing waves in the microwave frequency band or the millimeter wave frequency band, an oscillator for generating a high-frequency signal in that frequency band is necessary. When the high-frequency signal is to be frequency modulated (FM), as an oscillator of which the oscillation frequency can be variably controlled, for example a voltage controlled oscillator (VCO), whose oscillation frequency is variably controlled by means of an applied voltage, is used. FIGS. 20A, 20B, 21, and 22 show examples of conventional oscillator constructions. The basic construction of such an oscillator is shown in FIGS. 20A and 20B. FIG. 20A shows a band-pass type oscillator, and FIG. 20B shows a band-stop type oscillator.
As shown in FIGS. 20A and 20B, an oscillator 101 is made up of a negative resistance circuit 102 having a signal-amplifying action and a resonating circuit 103 which determines the oscillation frequency. In the negative resistance circuit 102, a feedback circuit wherein positive feedback is applied to an active device such as a transistor, or a device having negative resistance itself (for example a Gunn diode) is used. The resonating circuit 103 consists of, for example, a cavity resonator, a dielectric resonator or a plane resonator. The band-pass type oscillator 101 (FIG. 20A) is an oscillator in which the signal is taken from a resonating circuit 103 side, and the band-stop type oscillator 101 (FIG. 20B) is an oscillator in which the signal is taken from a negative resistance circuit 102 side.
In the oscillator constructed as described above, in the initial stage of oscillation a signal is passed back and forth between the negative resistance circuit and the resonating circuit, and by the signal being strengthened in the negative resistance circuit and a. frequency being selected in the resonating circuit a state of stationary oscillation at a set frequency is established. The output power at the time of the stationary oscillation depends on the amplifying capability of the negative resistance circuit, i.e. on the strength of the negative resistance. The strength of the negative resistance is generally evaluated using the resistance component of the impedance of the transistor seen from a point near the output terminal of the transistor in the feedback circuit of the negative resistance circuit. Because in an oscillator, normally, the higher the output power is the better, the feedback circuit is designed so that the negative resistance is a maximum.
In an oscillator of the construction described above, to variably control the oscillation frequency, all that is necessary is to change the frequency characteristics of either the negative resistance circuit or the resonating circuit. Here, an example of a voltage controlled oscillator (a band-stop type voltage controlled oscillator) is shown in FIG. 21. In the voltage controlled oscillator 101 shown in FIG. 21, a variable capacitance diode (hereinafter called a varactor) 104 is provided in the resonating circuit 103. By varying the capacitance of the varactor 104 by means of a voltage applied to a frequency control voltage terminal 105, the resonance frequency of the negative resistance circuit 102 is varied and the oscillation frequency of the voltage controlled oscillator 101 is thereby varied.
In this voltage controlled oscillator 101 using the varactor 104, by selecting a suitable varactor 104 according to the range over which the frequency is to be controlled, it is possible to set this frequency range relatively freely. However, when circuit miniaturization is attempted by making the whole voltage controlled oscillator 101 as a single integrated circuit, that is, when attempting to make it as a monolithic microwave integrated circuit (hereinafter abbreviated to MMIC), because the varactor 104 and the transistor (or Gunn diode) are devices made using different semiconductor film structures, it has been extremely difficult to make the whole voltage controlled oscillator 101 as an MMIC.
The construction of a voltage controlled oscillator, in which a varactor is not used and which is made as an MMIC, is disclosed in Japanese Patent Application Laid-Open No. S.62-207006. This voltage controlled oscillator made as an MMIC is shown in FIG. 22. FIG. 22 shows a band-pass type voltage controlled oscillator 101 wherein a transistor, for example a field effect transistor (hereinafter abbreviated to FET), 106 is provided in the negative resistance circuit 102. By varying the gate-source capacitance of the FET 106 by means of a gate bias voltage applied to the FET 106 through a frequency control voltage terminal 105 the resonance frequency of the resonating circuit 103 is varied and the oscillation frequency of the voltage controlled oscillator 101 is thereby varied.
In this construction, because transistors (FETs) made by using the same semiconductor film structure are used in both the negative resistance circuit 102 and the resonating circuit 103, it becomes possible to integrate the whole of the voltage controlled oscillator 101 onto a single semiconductor substrate, and making it as an MMIC becomes easy.
When a voltage controlled oscillator is used in a frequency modulation circuit, it is desirable that linearity (a proportional relationship) be maintained between the control voltage impressed on the voltage controlled oscillator and the oscillation frequency. Further, when frequency modulation is carried out using a voltage controlled oscillator, conventionally, the frequency modulation width has been of the order of a few MHz. As long as frequency modulation of this order is executed, there has been no problem in practice with the use of voltage controlled oscillators of the conventional constructions described above (oscillators using a varactor 104 or oscillators made as MMICs).
However, the present inventors envisaged setting the center frequency of the oscillation frequency range of a voltage controlled oscillator to a level of 30 GHz or 60 GHz and setting the frequency modulation width to several tens of MHz or more. Together with this, the inventors envisaged making a voltage controlled oscillator operated in this frequency band as an MMIC. To realize these objectives, the inventors produced by way of a trial a voltage controlled oscillator 111 of the circuit construction shown in FIG. 8. This voltage controlled oscillator 111 will now be described in detail. (FIG. 8 is a circuit diagram for illustrating a third preferred embodiment of the present invention, but because this trial-production voltage controlled oscillator 111 has the same circuit diagram as that shown in FIG. 8 it will be described using FIG. 8. Also, this trial-production voltage controlled oscillator 111 is not known technology at the time of application of the present invention.)
As shown in FIG. 8, the voltage controlled oscillator 111 consists of a negative resistance circuit 112 and a resonating circuit 113. The negative resistance circuit 112 is made up of as the transistor for example a high electron mobility transistor (hereinafter abbreviated to HEMT) 114, a transmission line 115 applying series feedback to the source of this HEMT 114, a matching circuit 116 and a D.C. cutoff capacitor 117. In this case, one end of the transmission line 115 is connected to the source of the HEMT 114 and the other end is grounded. The matching circuit 116 is made up of a transmission line 118, a stub 119 and a high-frequency grounding capacitor 120 connected in series.
One end of the transmission line 118 (the terminal on the opposite side from the terminal connected to the stub 119) is connected to the drain of the HEMT 114. The point of connection between the stub 119 and the high-frequency grounding capacitor 120 constitutes a voltage terminal 121 for supplying a drain bias. The other end of the high-frequency grounding capacitor 120 is grounded. Also, one end of the D.C. cutoff capacitor 117 is connected to the point of connection between the transmission line 118 and the stub 119, and the other end of this D.C. cutoff capacitor 117 constitutes an output terminal 122.
The resonating circuit 113 consists of a plane resonator made up of a transmission line 123 and a capacitor 124 connected in series. One end of the transmission line 123 (the terminal on the opposite side from the terminal connected to the capacitor 124) is connected to the gate of the HEMT 114. The point of connection between the transmission line 123 and the capacitor 124 constitutes a voltage terminal 125 for supplying a gate bias. This gate bias is a control voltage (i.e. a D.C. bias voltage) for controlling the oscillation frequency of the voltage controlled oscillator 111. The other end of the capacitor 124 is grounded.
The circuit elements (namely the HEMT 114, the transmission lines 115, 118 and 123, the stub 119 and the capacitors 117, 120 and 124) constituting the voltage controlled oscillator 111 described above are formed integrated on, for example, an InP substrate. In this way the voltage controlled oscillator 111 is made as an MMIC. This trial-production voltage controlled oscillator 111 is an MMIC for outputting a high-frequency oscillation signal in, for example, the 30 GHz band.
The HEMT 114 formed on the above-mentioned InP substrate is an HEMT in which an InAlAs/InGaAs pseudomorphic hetero-structure is used and of which the gate length is 0.5 .mu.m, the unit gate width is 13 .mu.m and the number of fingers is four. In making the MMIC, a coplanar line 126 of the construction shown in FIG. 9 was used for the transmission lines and the stubs. This coplanar line 126 is made up of a signal line 128 and ground electrodes 129 disposed on either side of this signal line 128, all disposed on an InP substrate 127. The signal line 128 and the ground electrodes 129 are for example made of gold. The width dimension Ws of the signal line 128 was made 50 .mu.m and the spacing Wg between the signal line 128 and the ground electrodes 129 was made 43 .mu.m. In this case, the wavelength of the 30 GHz high-frequency signal inside the coplanar line 126 was, according to calculation, about 3900 .mu.m.
Also, in making the trial-production voltage controlled oscillator 111 (MMIC) described above, the inventors designed the feedback circuit so that the strength of the negative resistance (i.e. the feedback strength) of the negative resistance circuit 112 was maximized. The reason for designing it in this way is to maximize the output power of the high-frequency signal outputted by the voltage controlled oscillator 111 and to stabilize the output.
The strength of the negative resistance was found by calculating on the basis of results obtained when the respective S parameters of the HEMT 114, the capacitor 120 and the transmission line 115 were measured. Specifically, the absolute value of the negative resistance component (.vertline.Re(Za).vertline., where Re(Za)&lt;0) was obtained by varying the length Lb of the transmission line 115 shown in FIG. 8 and calculating the impedance Za in a case where the HEMT 114 side is seen from the drain electrode of the HEMT 114, which is its output terminal.
As a result of this calculation, it was found that the negative resistance is at its strongest, i.e. the absolute value of the negative resistance component is at its greatest, when Lb=1121 .mu.m. The value of the negative resistance in this case was Re(Za)=-104 .OMEGA.. Accordingly, the inventors set the length Lb 20 of the transmission line 115 of the negative resistance circuit 112 to 1121 .mu.m and set the length of the transmission line 123 of the resonating circuit 113 and the lengths of the transmission line 118 and the stub 119 of the matching circuit 116 to lengths such that a high-frequency signal in the 30 GHz band would be outputted, and thereby made the voltage controlled oscillator 111 (MMIC).
The inventors then measured the voltage (gate bias) vs. oscillation frequency characteristic of this voltage controlled oscillator 111. In this case, the drain bias impressed on the voltage terminal 121 was set to 2.5 V. The oscillation frequency and output power were then measured while the gate bias impressed on the voltage terminal 125 was changed in small steps from 0.20 V to -0.30 V. At this time, in the gate bias voltage range of from 0.00 V to -0.20 V the gate bias was changed in particularly small steps of for example 0.01 V, and in the rest of the gate bias voltage range the gate bias was changed in steps of for example 0.05 V.
The measurement results are shown plotted on a graph in FIG. 10. In FIG. 10, the rhomboid points show the frequency characteristic and the circular points show the output power characteristic. From FIG. 10, the inventors discovered that the trial-production voltage controlled oscillator 111 made as described above has the characteristic that its oscillation frequency changes in steps with respect to change in the gate bias. It could also be seen that the output power is about 1 to 2 dBm and therefore the output is amply large (i.e. maximized).
However, the characteristic that the oscillation frequency changes in steps like this shows that linearity of the change in the oscillation frequency with respect to the control voltage (the gate bias) is not being maintained. Therefore, this trial-production voltage controlled oscillator 111 cannot be used in a frequency modulation circuit.
The inventors then looked for a way of constructing a voltage controlled oscillator Ill realized as an MMIC by being made in the way described above so that the D.C. bias voltage (gate bias) and the oscillation frequency have linearity. The inventors focused on the feedback strength of the feedback circuit in the negative resistance circuit 112 of the voltage controlled oscillator 111. The inventors supposed that when the feedback strength of the feedback circuit is at a maximum, because the stability of oscillation is at its highest (the Q value is at its greatest), the oscillation frequency changes less easily, or, in other words, the oscillation frequency is difficult to variably control. Developing this idea, the inventors raised the hypothesis that if the stability of oscillation were to be reduced so that the oscillation frequency changes more easily then the control voltage (gate bias) and the oscillation frequency might come to have linearity.
To test this hypothesis, the inventors carried out the experiment of making a voltage controlled oscillator 111 (MMIC) wherein the feedback strength of the feedback circuit is made less than the maximum. When the voltage (gate bias) vs. oscillation frequency characteristic of this voltage controlled oscillator 111 was measured, it was confirmed empirically that linearity of the change in oscillation frequency with respect to the gate bias is amply maintained. A specific construction and measured results of this voltage controlled oscillator 111 (MMIC) wherein linearity is amply maintained are discussed in detail later in the section on preferred embodiments of the invention.
Accordingly, a second object of the invention is to provide a voltage controlled oscillator of a circuit construction such that it can easily be made as an MMIC and constructed so that the D.C. bias voltage and the oscillation frequency have linearity.